1. Field of the Invention
The present invention relates generally to the field of semiconductor fabrication and, more particularly, to a method for reducing particles during ion implantation.
2. Description of the Prior Art
Ion implanters are commonly used in the manufacture of semiconductor products for implanting ions into semiconductor substrates to change the conductivity of the material in such substrates or in pre-defined regions thereof. A typical implanter consists of an ion source, a mass analyzer, a Faraday flag, an electron shower, and a disk assembly. The ion source supplies the ions to be implanted. These ions can be of several different chemical elements and pass through a magnetic field in the mass analyzer. Based on the mass to charge ratio of the generated ions, the mass analyzer selects certain ions to reach the target wafer for implantation.
The Faraday flag is a monitoring assembly for measurement and setup prior to implanting. Typically made of graphite, the Faraday flag is used to block the ion stream before implantation begins and is physically moved to allow the ions to reach the target wafer during implantation. In the closed position, the Faraday flag blocks the ion beam and implements direct measurement of beam currents.
Typically, the Faraday flag encompasses a graphite beam plate with a flat surface. When the Faraday flag is disposed in the aforesaid closed position, the ion beam impinges on the flat surface and the ion implanter begins to measure the beam current of the ion beam. During the measurement of the beam current, dopant species such as phosphorus, germanium or boron deposit and accumulate on the flat surface of the beam plate, and gradually forms a material film thereon. This is problematic because the material film deposited on the flat surface of the graphite beam plate becomes a contamination source.
According to the prior art, particles resulting from beam strike on the material film deposited on the flat surface of the graphite beam plate readily escape out of the Faraday flag and thus contaminate the ion beam. In a worst case, the accelerated particles chipped off from the flat surface of the graphite beam plate might severely damage the circuit features such as polysilicon gate lines formed on the semiconductor wafer.
In light of the above, a need exists for an improved ion implant method and an improved design of the Faraday flag that are capable of reducing particle problem and substantially eliminating damage to the wafer.